1. Field of the Invention
The invention relates to a current detecting method and device for a DC-DC converter. In particular, the invention relates to such a method and device for detecting a variation in a DC current being supplied to a load in supplying variable DC power to the load via an inductor from a synchronous rectification DC-DC converter in which a first switching element connected to one electrode of a DC input power source and a second switching element connected to the other electrode are turned on and off alternately in a prescribed period.
2. Background Art
In step-down-type synchronous rectification switching converters etc. overcurrent protection may be performed for a load, or a function may be implemented where the occurrence of switching to a light load (the output current is small) is judged and a transition automatically is made to a light load mode. In such instances it is necessary to detect, in a prescribed switching period, an average current being actually supplied to the load.
Conventional current detecting methods for a DC-DC converter commonly employ a technique wherein a resistor is inserted in an output current path and a potential difference between opposite ends of the resistor is detected and converted into a current value. However, this technique has a problem that the insertion of the resistor lowers the power conversion efficiency of the DC-DC converter. In view of this, in recent years, a technique has become common wherein a current variation is detected by detecting a variation in the voltage across a switching element, such as by utilizing the on-resistance of the switching element to detect that a transistor is in an on state.
However, the on-resistance of a transistor tends to be influenced by process variations, variation in environment temperature, etc. Further, where importance is attached to power conversion efficiency converters use transistors having a small on-resistance. This makes detection of a very small voltage variation necessary, and results in a problem of insufficient measurement accuracy.
In view of the above, a technique has been proposed that an on-resistance and a detection resistor are connected to each other serially to increase the accuracy of measurement of a voltage variation. However, in JP-A-2002-10627 (paragraphs [0011]–[0026] and FIGS. 1 and 2), power loss occurs in the detection resistor and hence the power conversion efficiency of the converter is lowered. Further, a measuring circuit is required to perform a measurement only when the transistor is on. This makes it difficult to realize a detection circuit for securing high measurement accuracy, particularly in a case that the switching frequency is high.